Radio is a technology that allows for the transmission of sound or other signals by modulation of electromagnetic waves.
A radio wave is created whenever a charged object accelerates.
When a radio wave passes a wire, it induces a moving electric charge that can be detected. Since radio seems to act at a distance, it provides many nearly magical modern techniques.

The theoretical basis of the propagation of electro-magnetic waves was first described by James Clerk Maxwell in his paper to the Royal Society A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865.

It was Heinrich Rudolf Hertz who, between 1886 and 1888 first validated Maxwell's theory through experiment, demonstrating that radio radiation had all the properties of waves, and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation.

On Christmas Eve, 1906, using his heterodyne principle, Reginald Fessenden transmitted the first radio broadcast in history from Brant Rock Station, Massachusetts[?]. Ships at sea heard a broadcast that included Fessenden playing the song O Holy Night on the violin and reading a passage from the Bible. The World's first regular wireless broadcasts for entertainment commenced in 1922 from the Marconi Research Centre at Writtle[?] near Chelmsford, England, which was also the location of the World's first "wireless" factory.

The identity of the original inventor of radio, at the time called wireless telegraphy, is contentious.

Guglielmo Marconi was awarded what is sometimes recognised as the World's first patent for Radio with BritishPatent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for.

In the USA, some key developments in Radio's early history were created and patented in 1897 by Nikola Tesla. However the US Patent Office reversed it's decision in 1904, awarding Guglielmo Marconi a patent for the invention of Radio, possibly influenced by Marconi's financial backers in the States, who included Thomas Edison and Andrew Carnegie. In 1909 Marconi, with Karl Ferdinand Braun were also awarded the Nobel Prize in Physics for "contributions to the development of wireless telegraphy".

However, Tesla's patent (number 645576) was reinstated in 1943 by the US Supreme Court, shortly after his death. This decision was apparently made for financal reasons, to allow the US Government to avoid having to the pay damages that were being claimed by the Marconi Company for use of it's patents during World War I.

Claims have also been made that Nathan Stubblefield[?], invented radio before either Tesla or Marconi, however his device seems to have worked by induction transmission rather than radio transmission.

Marconi opened the World's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people.

Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming.

There are a number of uses of radio:

Audio

The oldest form of audio broadcast was marine radio telegraphy, now no longer used. A continuous wave, or CW, was switched on and off by a key to create Morse code, which was heard at the receiver as an intermittent tone.

AM radio sends music and voice. AM radio uses amplitude modulation, in which higher air-pressure at the microphone causes higher transmitter power. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.

FM radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, a higher air-pressure at the microphone turns into a higher transmitted frequency. FM is transmitted as Very High Frequency radio waves (VHF). There are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that the shorter radio waves act more like light, and travel in straight lines that do not bend around the Earth.

FM Sub-band services transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes on unused space in an FM station's allocation. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.

Marine and aviation voice radios use VHF AM. This is good for aircraft and boats because the antennas are lightweight. Aircraft are often so high that their radios can see hundreds of miles, even though they are using VHF.

Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kilohertz of deviation (5 thousand cycles per second) for maximum pressure, rather than the 16 used by FM broadcasts and TV sound.

Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. SSB sounds like ducks quacking on an AM radio. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by sacrificing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.

Telephony

Cell phones transmit to a local cell radio, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones orginally used FM, but now most use various digital encodings.

Television sends the picture as AM, and the sourd as FM, on the same radio signal.

Digital television encodes three bits as eight strengths of AM signal. The bits are sent out-of-order to reduce the effect of bursts of radio noise. A Reed-Solomon error correction code lets the receiver detect and correct errors in the data. Although any data could be sent, the standard is to use MPEG-2 for video, and five CD-quality (44.1 kilo-sample/sec) digital channels (center, left, right, left-back and right back). With all this, it takes only half the bandwidth of an analog TV signal because the video data is compressed.

Navigation

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.

Loran systems also used time-of-flight radio signals, but from radio stations on the ground.

VOR[?] systems (used by aircraft), have two transmitters. A directional transmitter scans like a lighthouse at a fixed rate. When the directional transmitter is facing north, an omnidirectional ransmitter pulses. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.

Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators.

Radar detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.

Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft.

General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.

Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter.

Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more.

Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.

Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.

IEEE 802.11, the radio network standard, has stations with digital tuners. They start off by contacting a central control node, which tells the nodes about each other so they can communicate privately. Nodes move through many frequencies. They use a pseudo-random number generator to select the next frequency.

Radio teletypes usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. These are classically used by the military and weather services.

Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data.

Heating

Microwave ovens use intense radio waves to heat food. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.)

Mechanical Force

Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments.

Space drive: Radiation pressure from intense radio waves has been proposed as a propulsion method for interstellar probes. Since the waves are long, the probe could be a very light-weight metal mesh, and thus achieve higher accelerations than if it were a light sail.

Other

Amateur radio is an emergency and public-service radio service provided by enthusiasts who purchase or build their own equipment. It operates in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.